1. Field of the Invention
This invention relates generally to stabilization systems. More particularly, this invention relates to an accelerometer based stabilization apparatus that operates to stabilize an object such as a sensing or imaging device in a vehicle, such as an airplane, while the vehicle is subject to rotational and translational motion.
2. Description of Related Art
Sensing equipment, such as electronic imaging devices, cameras, radar, navigation instruments, and the like are frequently carried by and operated in a moving vehicle, such as an airplane, that undergoes rotational motions about its center of rotation. In such an environment, the equipment may be mounted on a movable platform that is stabilized with respect to movement of the vehicle. In some applications, such platforms are stabilized about only one axis. In other applications, the platforms are stabilized about two or three axes. These axes correspond to the roll, pitch, and yaw axes of the vehicle. Particular applications involve the stabilization of the line-of-sight of a camera or other imaging device. In this case, when the vehicle undergoes rotational motion about its axes, the line-of-sight remains fixed with respect to the inertial reference frame of the vehicle.
The prior art platform stabilization systems are based on the operation of the gyroscope. Typically mounted on gimbals, gyroscopes are used to absorb torques and provide inertial stiffness to the platform by exerting counter rotations on the gimbal. In other applications, gyroscopes are used in a control system where the tendency of the gyroscope to undergo precession is detected by a pickoff, which passes the signal to a motor that provides a compensating torque to the platform to stabilize it. See, Avionics Navigation Systems, Kayton, Fried Ed., p. 394 to 405.
Linear accelerometers are devices that measure bidirectional accelerations along an axis. It is known in the art that combinations of linear accelerometers, when arranged in various orientations, can be used to measure angular accelerations. See, e.g., A. S. Hu, Rotational Measurement Technique Using Linear Accelerometers, ISA (1977); V. I. Lopatin, Measurement of Absolute Angular Velocity of Aircraft by Means of Linear Accelerometers (1966). Alternatively, the angular accelerations of a body can be obtained at higher cost by angular accelerometers. Linear accelerometers have been used in inertial measurement units, such as the one described in U.S. Pat. No. 4,711,125, and in inertial navigation units. Linear accelerometers have been incorporated into stabilization systems that are founded upon the operation of the gyroscope. U.S. Pat. No. 3,936,716 discloses a 2 perpendicular axis stabilization platform stabilized by means of a gyro-controlled servo system mounted in a ship. In this patent, the linear accelerometers are used to hold a gyro in a vertical position, the linear accelerometers being sensitive to changes in speed and course of the ship.
Gyroscope based platform stabilization systems have numerous disadvantages, principally high cost and added weight. In some cases, the cost of gyroscopes and their associated equipment is the principal component to the overall cost of a stabilized platform.
Hence, an object of the invention is to provide gyroless platform stabilization techniques capable of stabilizing a sensing device or other object about one or more axes of rotation of the vehicle.
A further object of the invention is to provide platform stabilization techniques capable of scanning applications whereby the sensing device or object is not only stabilized but also scans according to externally supplied scanning criteria. Additionally, it is an object of the present invention to provide platform stabilization techniques capable of slewing operations whereby the sensing device of object is not only stabilized, but also slews to a desired orientation according to externally supplied commands.
A further object and advantage of the invention is that the stabilization apparatus is a light weight, low cost system, suitable for those applications where size and weight are significant restraints. The present invention has additional advantages in that it is a robust system and inherently more reliable than gyroscope systems, and generally has less power consumption.
A still further advantage of the invention is that it is based on accelerometers, which determine the angular acceleration of the stabilized platform mounting base (attached to the vehicle), or of the sensing device or object depending on the configuration used, and which also measure the cross product of the angular velocities and provide the necessary information for the computation of the statistics of the angular motions as a stochastic process. Accurate knowledge of such statistics is significant when optimal state estimators, such as Kalman Filters, are used to estimate the true value of the angular motion.